The invention concerns a cryostat with a first helium tank which contains helium at an operating temperature T1<3 K, and a second helium tank which is connected to the first helium tank, and contains liquid helium at an operating temperature T2>3 K, wherein the first helium tank has a cooling means which generates an operating temperature T1<3 K in the first helium tank, wherein the cooling means is designed as a Joule-Thomson valve with downstream heat exchanger and supplies pumped helium to a room temperature region outside of the cryostat. A magnet system of this type is disclosed in U.S. Pat. No. 5,220,800.
Superconducting magnet systems of this type generally comprise a cryostat with two chambers, with a superconducting magnet coil being disposed in the first chamber. The second chamber serves as a helium supply which is under atmospheric pressure or slight overpressure at a temperature of approximately 4.2 K. The two chambers communicate and helium can flow from the upper into the lower chamber where it is cooled to a temperature considerably below 4.2 K using a further cooling unit which projects into the first chamber. A radiation shield reduces the impinging radiation energy and includes a tank which is filled with a cryogenic liquid and cools the radiation shield.
There are conventional further cooling units to further cool the liquid helium in the first chamber, wherein the helium is relaxed through a needle valve to a low pressure and is pumped out of the first chamber. Disadvantageously, the pumped helium is removed from the system such that the second chamber, which is connected to the first chamber, is slowly emptied and the helium in the second chamber must be replaced at regular intervals.
Refrigerators are conventionally used to cool the helium in cryostats with one chamber or also to cool radiation shields, wherein a working gas is expanded or compressed using a piston motion (piston refrigerator). Disadvantageously, the permanent piston motion generates vibrations and also causes magnetic disturbances in the main magnetic field of the coil due to the metallic piston. The motion of the piston at the cold end of the refrigerator is also problematic, since lubrication is not possible due to the low temperature, resulting in the need for frequent maintenance.
In contrast thereto, pulse tube coolers effect expansion or compression of the working gas using a shock wave front in a pulse tube. The shock wave front is thereby controlled by a suitable valve arrangement, usually by a rotating valve. The pulse tube is connected to a regenerator in which heat exchange between the working gas and the regenerator material is provided. After compression of the working gas, the gas flows through the regenerator and is then relaxed in the expansion chamber. The gas which is cooled thereby accepts heat from the surroundings of the expansion chamber thereby cooling those surroundings. Since the rotating valve must not be disposed in the direct vicinity of the magnet system, a pulse tube cooler represents a smoothly running, low-wear cooling means which avoids moving parts in the low-temperature region.
In the magnet system of U.S. Pat. No. 5,220,800, the further cooling unit which projects into the first chamber pumps liquid helium to further cool the liquid helium bath in the first chamber via expansion. A disadvantage of this arrangement is that the refrigerator permanently consumes helium. This is particularly disadvantageous, since measurements must be interrupted to refill the helium and this involves considerable expense. Moreover, helium is not always available in arbitrary amounts. For this reason, it is desirable to reduce the helium consumption of a magnet arrangement of this type.
It is therefore the underlying purpose of the invention to present a superconducting magnet system which minimizes the helium consumption thereby omitting unnecessary interruption of measurements due to frequent refilling of helium.